TMZ Hip Hop obtained an open letter Tuesday, penned by PETA's Senior VP Lisa Lange and addressed to Pharrell, which reads ... "Dear Pharrell, This is an invitation the likes of which you’ve never been offered before."

The correspondence is filled with references to Pharrell-produced hits, yet there's nothing joking about it as Lange says ... "When you were made men’s creative director of Louis Vuitton, we hoped animals would get lucky and that you would choose vegan fabrics, which are the preference of today’s ethical and environmentally conscious consumers. But we aren’t happy to note your newest design, the "Millionaire" Speedy bag, which is crafted from crocodile skin, because when it comes to animals being used for fashion, it isn’t beautiful—it’s abuse."

Lange also accused the Neptunes' superproducer of sitting idle as the crocs get mutilated for their skin -- a complaint she also recently leveled at Offset for his birthday bag blowout for Cardi B.

The letter continued, "A PETA Asia investigation found workers at LVMH’s suppliers cutting live crocodiles’ necks open and ramming metal rods down their spines while they were still conscious. Another investigation revealed that workers at facilities supplying LVMH struck pythons repeatedly on the head, suspending them in the air, inflating their bodies with water, and disemboweling them -- even as they moved about."

Lange wants Pharrell to go to a crocodile factory farm, where she says bags like the Millionaire Speedy are made ... and she's warning it won't be a fun trip to the zoo.

In her words -- "You’d want to bring along nose plugs and high boots to wade through fetid, waste-filled water. If we go to an indoor tank, also bring a flashlight, because you won’t see daylight. There are no blurred lines here. Killing wildlife for a bag isn’t cool—it’s cold. Are you up for this trip?"